This invention relates to a process for producing an array-type ultrasonic probe using a polymeric piezoelectric film as an ultrasonic transducer element.
Ultrasonic transceivers (transmitter-receivers) have heretofore been widely used, for example, in depth sounders, fish sounders, and ultrasonic detectors. Recently, the application of ultrasonic transceivers to medical diagnostic equipment has been rapidly progressing. The ultrasonic transceiver for medical diagnosis is operated on the principle that an ultrasonic wave generated by the ultrasonic probe is reflected at boundaries between portions of a living body having different acoustic impedances (velocity of sound.times.density), and the resultant ultrasonic echo received by the ultrasonic probe is subjected to signal-conditioning and displayed on a cathode-ray tube. In the ultrasonic wave generating part of such an ultrasonic probe, a vibrating member comprising a piezoelectric element is used. In order to improve the resolution of sectional plane images in a deep portion of a living body, a higher frequency of ultrasonic wave is gradually required. For complying with this trend, an array-type probe is preferred, wherein the piezoelectric element is divided into a number of small and thin unit elements. The array-type probes are generally classified, according to the arrangement of unit piezoelectric elements, into those of the annular-type wherein fine unit elements having shapes of annular rings with gradually different diameters are radially arranged with a small gap therebetween, and those of the linear-type wherein linear or thin bar-shaped unit elements are arranged in parallel with each other with a small gap therebetween.
Among these, as annular-type probes, for example, those having detailed structures as shown in FIGS. 1(a) and 1(b) or FIGS. 2(a) and 2(b) have heretofore been used wherein FIGS. 1(a) and 2(a) represent partial sectional elevations and FIGS. 1(b) and 2(b) plan views.
A conventional process for producing an annular probe as shown in FIGS. 1(a) and 1(b) will now be explained along with its structure. On a concave upper face of a substrate 1 made of plastics, etc., a metal reflection plate 2 and a ceramic piezoelectric plate 3 having a back electrode 4 and a front electrode 5, each having a curvature corresponding to the concave upper face, are successively disposed by bonding. Thereafter, annular ring-shaped grooves 6 are formed by cutting the laminate by means of a diamond cutter, wire saw, laser beam or abrasive member to leave separate transducer elements each bonded to the substrate 1. Herein, the resultant annular ring-shaped elements are caused to have substantially the same surface areas by gradually changing the widths thereof from the central one to the peripheral one so that the respective ring elements will emit substantially the same energy of ultrasonic waves, while they are drawn to have apparently similar widths in FIGS. 1 through 5 for convenience of drawings.
On the other hand, the structure shown in FIGS. 2(a) and 2(b) is formed by bonding a metal reflection plate 2a onto a substrate 1, forming annular ring-shaped grooves 6a by cutting only the reflection plate 2a, and forming thereon a ceramic piezoelectric plate 3 having only a front electrode 5.
However, the above explained conventional production processes, because of minute processing by cutting involved therein, accompany several disadvantages as follows. Thus, they are time-consuming. Cooling water, cooling oil or other coolants give ill effects to both post-processing and product characteristics. The cutting operation sometimes results in separation between the substrate and the reflection plate or between the reflection plate and the piezoelectric member, complete or partial separation, breakage or fracture of the reflection plate or the piezoelectric plate, and poor insulation at separating grooves or lowering or limitation in accuracy of working of separating grooves due to incomplete removal of cutting residues. Further, when a polymer dielectric film is used in place of the ceramic dielectric plate in the above mentioned conventional processes in order to comply with the requirement of a thin piezoelectric member, the piezoelectric performance of the polymer piezoelectric film is deteriorated due to cutting heat from cutting. These circumstances are not peculiar to the production of annular array-type probes but hold true with the production of the array-type ultrasonic probes in general including those of linear arrangement, wherein small unit piezoelectric elements are arranged with small gaps or intervals on a substrate.
In order to remove the difficulties as mentioned above accompanying the cutting of a reflection plate bonded onto a substrate, it may be conceivable to produce a structure shown in FIGS. 2(a) and 2(b) by producing individual units of back electrode and reflection plate 2a in advance by cutting and the like processing, and bonding them one by one onto the upper face of the substrate 1. One difficulty with this approach is that the structure of FIG. 2, when compared with that of FIG. 1, tends to give a lower element division or separation performance by separating grooves 6a and result in poor performances because of the use of a uniformly extending front surface electrode, especially in the case where the piezoelectric member 3 is thick, as is the case with a ceramic piezoelectric member. Moreover, in the case of an array-type ultrasonic probe for use in a medical diagnostic instrument, an extremely accurate arrangement is required such that reflection plates having a 1 mm-width, for example, are arranged with a uniform gap of 0.05 mm therebetween, and it is almost impossible to perform such an accurate arrangement by successive bonding of unit reflection plates. This is particularly true when the upper surface of a substrate 1 to which the reflection plates are applied are made concave in order to enhance the ultrasonic transmitting and receiving performance.
As another approach, a structure of FIG. 2 may be formed by bonding a uniform layer of reflection plate 2a onto a substrate 1 and thereafter etching the reflection plate 2a to form separating grooves 6a therein. In this case, however, there are accompanied several problems such as deterioration of the adhesive by the etchant, complete or partial separation of the reflection plate, and lowering in accuracy of working of separating grooves due to over-etching, undercutting, etc.